The Huari Huari deposit, Potosí Department in SW Bolivia, hosts polymetallic stratiform and vein mineralization of Miocene age with significant concentrations of the critical metal indium (In). Vein mineralization records document early crystallization of quartz and cassiterite followed by prominent associations of sulfides and sulfosalts. The earliest sulfide was arsenopyrite, followed by pyrrhotite, and progressively giving way to pyrite as the main iron sulfide, whereas Cu–Ag–Pb sulfosalts constitute late hypogene associations. Sphalerite is the chief ore mineral, and its crystallization is extended during most of the mineralization lifespan as evidenced by its initial cocrystallization with pyrrhotine, then with pyrite, and finally with Ag–Pb sulfosalts. The composition of sphalerite varies from early to late generations with a continuous decrease in FeS that attests to a decrease in temperature, which is constrained to vary from ~450 to <200 °C, and/or an increase in f(S2), both congruent with the described paragenetic sequence. Indium concentrated mostly in the structure of Fe-rich sphalerite (up to 3.49 wt. %) and stannite (up to 2.64 wt. %) as limited solid solutions with roquesite in the (Zn,Fe)S–Cu2FeSnS4–CuInS2 pseudoternary system. In sphalerite, In shows a strong positive correlation with Cu at Cu/In = 1, suggesting its incorporation via a (Cu+ + In3+) ↔ 2Zn2+ coupled substitution, and it does not correlate with Fe. In stannite, In shows a moderate, negative correlation with Cu and Sn, and an In3+ ↔ (Cu+ + ½ Sn4+) coupled substitution is suggested. Coexisting sphalerite and stannite yielded the highest In concentrations and crystallized at temperatures between 350 and 250 °C. Copper activity probably played a major role in the accumulation of In in the structure of sphalerite since In-bearing sphalerite coexisted with the deposition of stannite, shows high concentrations of Cu (up to 0.13 atoms per formula unit (a.p.f.u.)) in its structure, and hosts exsolutions of stannite and chalcopyrite. Distribution on the district scale of In suggests an input of hydrothermal fluids richer in Cu in the central position of the mineralizing system, represented by the Antón Bravo vein.

The origin of the assemblage of ultra-high pressure (UHP), super-reduced (SuR) and several crustally derived phases in ophiolitic chromitites is still hotly debated. In this paper, we report, for the first time, this assemblage of phases in ophiolitic chromitites of the Caribbean. We studied the Mercedita chromitite deposit in the eastern Cuban ophiolitic complexes. The mineral phases were characterized using microRaman spectroscopy, energy-dispersive spectroscopy with a scanning electron microscope (SEM-EDS), X-ray microdiffraction and electron microprobe analyses. Mineral concentrates were prepared using hydroseparation techniques. We have identified oriented clinopyroxene lamellae in chromite, oriented rutile lamellae in chromite, moissanite hosted in the altered matrix of the chromitite, graphite-like amorphous carbon, corundum and SiO2 hosted in healed fractures in chromite grains, and native Cu and Fe–Mn alloy recovered in heavy-mineral concentrates obtained by hydroseparation. This assemblage may correspond to UHP-SuR conditions, implying recycling of chromitite in the mantle or formation of the chromite grains at deep mantle depths, followed by emplacement at a shallow level in the mantle. However, the chromitite bodies contain gabbro sills oriented parallel to the elongation of the chromitite lenses, and these show no evidence of HP/UHP metamorphism. Therefore, the identified “exotic” phases may not be indicative of UHP. They formed independently as oriented clinopyroxene lamellae in chromite during cooling (clinopyroxene and rutile), in super-reduced microenvironments during the serpentinization processes, and by transference of subducted crustal material to the mantle wedge via cold plumes.

Diamond is commonly regarded as an indicator of ultra-high pressure conditions in Earth System Science. This canonical view is challenged by recent data and interpretations that suggest metastable growth of diamond in low pressure environments. One such environment is serpentinisation of oceanic lithosphere, which produces highly reduced CH 4-bearing fluids after olivine alteration by reaction with infiltrating fluids. Here we report the first ever observed in situ diamond within olivine-hosted, CH 4-rich fluid inclusions from low pressure oceanic gabbro and chromitite samples from the Moa-Baracoa ophiolitic massif, eastern Cuba. Diamond is encapsulated in voids below the polished mineral surface forming a typical serpentinisation array, with methane, serpentine and magnetite, providing definitive evidence for its metastable growth upon low temperature and low pressure alteration of oceanic lithosphere and super-reduction of infiltrated fluids. Thermodynamic modelling of the observed solid and fluid assemblage at a reference P-T point appropriate for serpentinisation (350°C and 100 MPa) is consistent with extreme reduction of the fluid to logfO 2 (MPa) = −45.3 (ΔlogfO 2 [Iron-Magnetite] = −6.5). These findings imply that the formation of metastable diamond at low pressure in serpentinised olivine is a widespread process in modern and ancient oceanic lithosphere, questioning a generalised ultra-high pressure origin for ophiolitic diamond.

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Boletín de la Sociedad Geológica Mexicana / 2019 / 295 RESUMEN El Complejo Minero Neolítico de Gavà (CMNG) ubicado en Cataluña tuvo como objeto la explotación de la variscita verde utilizada para la elaboración de adornos y joyería. Los trabajos arqueológicos en más de 100 minas indican que la minera en este complejo se llevó a cabo ~5800 años antes del presente. CMNG constituye (A) uno de los primeros ejemplos conocidos de minería subterránea en Europa, (B) posiblemente el ejemplo más antiguo de minería a gran escala para uso ornamental y (C) el ejemplo más antiguo de aplicación de conceptos geológicos y mineros complejos. En el CMNG, la variscita se encuentra en dos estilos distintos: (1) reemplazamientos de láminas delgadas de fluorapatito intercaladas con pizarras negras ricas en minerales orgánicos de edad silúrica y (2) vetillas que cortan transversalmente estos materiales. El conjunto está cubierto discordantemente y localmente por caliches del Cuaternario. La variscita se formó como resultado de procesos supergénicos cuaternarios que oxidaron la pirita produciendo soluciones ácidas y oxidadas. Los fluidos resultantes movilizaron fosfato de apatito y lixiviaron Al y materia orgánica de las pizarras, lo cual condujo a la precipitación de los fosfatos ricos en Al como rellenos de venas y de reemplazamientos estratoligados de apatito. La formación de variscita se restringió a la extensión del frente de oxidación, y la variscita de ambos estilos de mineralización varia de color verde amarillento, en superficie, hasta verde profundo, en profundidad. El área de estudio contiene dos campos mineros: Can Tintorer y Can Badosa-Les Ferreres. La presencia de abundantes vetas subverticales de variscita en Can Tintorer permitió un complejo desarrollo de galerías en diferentes niveles de explotación comunicadas por pozos y rampas, logrando 15 metros de profundidad. La explotación empleada por los mineros neolíticos fue mediante realce ascendente y descendente; también usaron pilares y rellenos de explotaciones más antiguas para evitar el colapso de la mina. Estas minas fueron abiertas en muchos casos por pozos que fueron cavados directamente en duros caliches cuaternarios, favoreciendo la estabilidad de las galerías y revelando que los mineros comprendían la continuidad de la mineralización por debajo de la cubierta de caliches. El mapeo geológico revela que las explotaciones en el área de Can Badosa-Les Ferreres son simples, mostrando una entrada única a galerías simples o rampas que alcanzan menos de 5 m de profundidad y excavadas directamente en afloramientos de fosfatos. En contraste, las minas en el área de Can Tintorer tienen varias entradas y consisten en pozos verticales a través de una cubierta cuaternaria, presentando una intrincada geometría con muchas galerías grandes y comunicadas a diferentes profundidades con un sistema de cámaras y pilares. Aunque el desarrollo de ambas áreas mineras fue esencialmente contemporáneo, Can Badosa-Les Ferreres puede considerarse como un área de exploración. Además, la minería...